The proposed research aims to develop a comprehensive and spatially realistic computational model of action potential triggered, calcium dependent neurotransmitter release from synaptic vesicles. This model will provide unprecedented spatio-temporal insights into the physiological processes following the invasion of an action potential into a nerve terminal. Model development will proceed from an existing, spatially realistic baseline model of a single active zone, the specialized region within a nerve terminal that includes neurotransmitter release machinery. All simulations will be carried out with the program MCell which uses Monte Carlo algorithms to simulate molecular diffusion of calcium ions and buffer molecules, binding of calcium ions to receptors, and calcium channel gating inside arbitrary complex cellular spaces. Guided by experimental input, we will investigate the effect of perturbations and combinations thereof to the existing baseline model such as changes in the location, arrangement (homogeneous, randomized, clustered), and number of calcium channels;drug treatments (3,4-diaminopyridine, roscovitin) influencing the calcium channel kinetics and action potential shape;toxin treatments blocking fractions of calcium channels;and the effect of mobile buffer and/or mixtures of static and mobile buffer on vesicle release. These studies will lead to significant improvements of the baseline model and, thereby, set the stage for an investigation of short term synaptic plasticity via paired pulse action potentials. We will first identify mechanisms that give rise to the experimentally observed values for paired pulse facilitation and then investigate the effect of changes in the interstimulus interval and drug treatments on paired pulse facilitation. Clearly, the importance of elucidating the key elements of paired pulse facilitation and thereby synaptic plasticity for a more comprehensive understanding of human neural function can not be underestimated. The proper and faithful transmission of nerve signals along neurons in the human body is crucial for survival. Synapses constitute the connecting elements between neurons and cells they innervate and many debilitating neurological disorders are caused by synaptic dys-function. Hence, a proper understanding of synaptic function is crucial for the development of clinical treatments for neurological diseases. The proposed research will directly contribute to our understanding of synaptic function and is therefore directly relevant to the mission of the NIH.